1. Field of the Invention
This invention relates to electrical circuit overcurrent protection.
2. Introduction to the Invention
Positive temperature coefficient (PTC) circuit protection devices are well known. The device is placed in series with a load, and under normal operating conditions is in a low temperature, low resistance state. However, if the current through the PTC device increases excessively, and/or the ambient temperature around the PTC device increases excessively, and/or the normal operating current is maintained for more than the normal operating time, then the PTC device will be xe2x80x9cripped,xe2x80x9d i.e. converted to a high temperature, high resistance state such that the current is reduced substantially. Generally, the PTC device will remain in the tripped state, even if the current and/or temperature return to their normal levels, until the PTC device has been disconnected from the power source and allowed to cool. Particularly useful PTC devices contain a PTC element which is composed of a PTC conductive polymer, i.e. a composition which comprises (1) an organic polymer, and (2) dispersed, or otherwise distributed, in the polymer, a particulate conductive filler, preferably carbon black. PTC conductive polymers and devices containing them are described, for example in U.S. Pat. Nos. 4,237,441, 4,238,812, 4,315,237, 4,317,027, 4,426,633, 4,545,926, 4,689,475, 4,724,417, 4,774,024, 4,780,598, 4,800,253, 4,845,838, 4,857,880, 4,859,836, 4,907,340, 4,924,074, 4,935,156, 4,967,176, 5,049,850, 5,089,801 and 5,378,407, the disclosures of which are incorporated herein by reference for all purposes.
In a batch of PTC devices made by the same manufacturing process, uncontrollable variations in the process can cause substantial variation in the conditions which will trip any individual device. The largest steady state current which will not cause any of the devices in the batch to trip is referred to herein as the xe2x80x9cpass currentxe2x80x9d (IPASS) or xe2x80x9chold currentxe2x80x9d, and the smallest steady state current which will cause all of the devices to trip is referred to as the xe2x80x9ctrip currentxe2x80x9d (ITRIP). In general, the difference between IPASS and ITRIP decreases slowly as the ambient temperature increases. Depending on the particular type of device, ITRIP may for example be 1.5 to 2.5 times IPASS at 20xc2x0 C. For any individual device, the pass current and the trip current are the same. However, in this specification, reference is made to a PTC device having an IPASS and a different ITRIP, because as a practical matter, the manufacturer of an electrical switch must make use of PTC devices taken from a batch of such devices. Generally, the higher the ambient temperature, the lower the pass current and the trip current. This phenomenon is referred to as xe2x80x9cthermal deratingxe2x80x9d, and the term xe2x80x9cderating curvexe2x80x9d is used to denote a graph of temperature against pass current.
A limitation on the known uses of PTC protection devices is that when a PTC device is placed in series with the load and sized to conduct the normal circuit current, the PTC device can take a relatively long time to convert to its tripped state on an overcurrent which is, e.g., up to a few times the normal circuit current.
The invention provides a new overcurrent protection system which will give a rapid response to even relatively small overcurrents. In the new system, a sensor element and circuit interruption element are placed in series with the load. The sensor element is functionally linked to the circuit interruption element via a control element, so that, when the current in the circuit exceeds a predetermined amount, the sensor element senses the overcurrent and communicates with the control element. The control element causes the circuit interruption element to change from a relatively conductive normal state to a relatively non-conductive fault state (including a completely open state).
In an example of a preferred embodiment of circuit arrangements of the invention, the sensor element comprises a resistive device connected in series with the load, and the control element comprises a PTC device which is thermally linked to the resistive device and is electrically connected to the circuit interruption element. When an overcurrent passes through such a system, the resistive device increases in temperature causing the PTC device to heat up and trip to its high resistance state. The PTC device is linked to the circuit interruption element so that the increased resistance of the PTC device causes the circuit interruption element to switch into its fault state. The PTC device is not placed in series with the load and therefore may operate at current levels much less than the normal circuit current which passes through the load.
The thermal linking of a resistive device with a PTC device is known in the art. A current to be measured and/or controlled passes through the resistive device. I2R heating of the resistive device causes the PTC device to heat up and its resistance increases accordingly. Such resistive devices may comprise resistors, heaters, high resistance wire (e.g. NiChrome), PTC devices and the like. It is known that in order to obtain the desired current/temperature performance of such combinations, certain characteristics of the resistive device must be controlled, particularly in the zone adjacent to the PTC device. Some of the characteristics to be controlled include the resistivity, shape and cross sectional area of the material. The resistive device should be chosen to minimize system impedance while achieving sufficient temperature rise under overcurrent conditions to cause the PTC device to heat up and trip to its high impedance state.
In a first aspect, this invention provides an electrical protection system which can be connected between an electrical power supply and an electrical load to form an operating circuit, the operating circuit having an on state and an off state and comprising a current carrying line and a return line, and which when so connected protects the circuit from overcurrents, the system having a normal operating condition and a fault condition, and comprising:
a. a circuit interruption element, which, when the system is so connected, is connected in series between the power supply and the load, and has
(1) a closed state which permits the flow of a normal current, INORMAL when the system is in the normal operating condition, and
(2) an open state which permits the flow of at most a reduced current, substantially less than INORMAL when the system is in the fault condition;
b. a sensor element, which, when the system is so connected, is connected in series with the circuit interruption element and the load, and has
(1) a normal state, when the current in the system does not exceed the normal current, INORMAL by a predetermined current amount, and
(2) a fault state, when the current in the system exceeds the normal current, INORMAL, by the predetermined amount; and
c. a control element, which, when the system is so connected, is coupled with the sensor element and with the circuit interruption element, and has a variable resistance which
(1) is low when the sensor element is in the normal state, and
(2) increases by at least a predetermined resistance amount when the sensor element is in the fault state;
the circuit interruption element changing from its closed state to its open state, thereby causing the system to change from its normal operating condition to its fault condition, when the resistance of the control element has increased by the predetermined resistance amount in response to the sensor element changing from its normal state to its fault state.
In a second aspect, the invention provides an electrical protection system which can be connected between an electrical power supply and an electrical load to form an operating circuit, the operating circuit having an on state and an off state and comprising a current carrying line and a return line, and which when so connected protects the circuit from overcurrents, the system having a normal operating condition and a fault condition, and comprising:
a. a circuit interruption element, which, when the system is so connected, is connected in series between the power supply and the load, and has
i. a closed state which permits the flow of a normal current, INORMAL when the system is in the normal operating condition, and
ii. an open state which permits the flow of at most a reduced current, substantially less than INORMAL when the system is in the fault condition;
b. a sensor element, which has a variable resistance, and which when the system is so connected, is connected in series with the circuit interruption element and the load, and has
i. a normal state, in which its resistance is low, when the current in the system does not exceed the normal current, INORMAL, by a predetermined current amount, and
ii. a fault state, in which its resistance increases by at least a predetermined resistance amount, when the current in the system exceeds the normal current, INORMAL by the predetermined amount; and
c. a control element, which, when the system is so connected, is coupled with the sensor element and with the circuit interruption element, which causes the circuit interruption element to change from its closed state to its open state when the sensor element changes from its normal state to its fault state.
It will be apparent that polymeric PTC devices, ceramic PTC devices, other PTC devices such as bimetal devices, metallic PTC devices, arrangements of solid state devices with PTC characteristics, and devices displaying similar characteristics may be used in the circuit arrangements of this invention to provide reliable overcurrent protection. It will likewise be apparent to those of ordinary skill in the art that mechanical switches used in the circuit arrangements of this invention may include switches, relays, circuit breakers, isolators, bimetal devices and other devices. In addition, a solid state device or combination of solid state devices which provide disconnecting characteristics similar to those provided by mechanical switches may be used in place of the mechanical switches. Bimetal devices have also been referred to as bimetallic devices, electrothermal relays, thermally activated switches and/or electrothermal mechanisms with bimetal elements.
It will be apparent that in the preferred embodiments, this invention permits the use of PTC devices and bimetal switches to be arranged with mechanical switches and other electrical devices to provide reliable protection which protection was not previously available in the art. These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiments of the invention as illustrated in the various drawing figures.